1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film or a crystalline silicon film formed on an insulating substrate of glass or the like or promoting crystalline performance thereof by performing laser annealing thereto.
2. Description of Prior Art
In recent years researches have widely been carried out on the technology of crystallizing an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film having crystalline performance constituted not by single crystal but polycrystal, microcrystal or the like), that is, a non single crystal silicon film formed on an insulating substrate of glass or the like or promoting the crystalline performance by performing laser annealing in respect of the film. A silicon film is frequently used for the semiconductor film.
Compared with a quartz substrate which has been frequently used conventionally, a glass substrate has an advantage where the substrate is inexpensive and superior in fabrication performance and a substrate having a large area can easily be formed. Further, laser is preferably used in crystallization process since the melting point of the glass substrate is low. Laser can impart high energy only to a non single crystal film without considerably changing temperature of a substrate.
A crystalline silicon film formed by performing laser annealing is provided with high mobility. Further, researches have been carried out on the technology of forming a thin film transistor (TFT) by using the crystalline silicon film. According to the technology, a liquid crystal electro-optic device of a monolithic type where TFTs for pixel driving and for drive circuit are fabricated on one sheet of a glass substrate, can be provided. The crystalline silicon film is constituted by a number of crystal grains and therefore, the film is referred to generally as a polycrystalline silicon film or a polycrystalline semiconductor film.
Further, a method of performing laser annealing by fabricating a pulse laser beam of an excimer laser or the like having a large output into a square spot of several cm square or a linear shape of several mm width×several tens cm on an irradiated face by an optical system and scanning the laser beam (moving an irradiation position of the laser beam relatively in respect of the irradiated face), is preferably used since the method is provided with excellent mass production performance and is industrially excellent.
Particularly, when the linear laser beam is used, high mass production performance can be provided since laser irradiation can be carried out over the entire irradiated face by scanning the laser only in a direction orthogonal to the line direction different from a case of using a laser beam in a spot-like shape where scanning in the forward and rearward direction and in the left and right direction is needed.
Several problems have been posed in performing laser annealing in respect of a non single crystal silicon film by scanning a laser beam of a spot-like shape or a linear shape with a pulse laser beam as a light source.
A particularly serious problem is nonuniformity of effect of laser irradiation in a substrate face. As feature of laser beam, although provision of large energy is pointed out as the most preferable advantage, on the other hand, the pulse laser is provided with a drawback where dispersion of energy for respective shots of pulses is as large as several percent. According to the drawback, when, for example, a liquid crystal display is formed by crystallizing an amorphous silicon film by an excimer laser, there causes an inconvenience where trace of pulse of laser is visualized as it is on picture image.
Such an image failure constitutes a serious drawback in the present age where beautiful picture image is needed. The present invention has been carried out with an object of making inconspicuous or completely eliminating the drawback.